This invention relates to particle beams, and in particular, to systems and methods for imaging a cross-section of a semiconductor substrate using a particle beam.
Integrated circuits frequently incorporate devices that extend deeply into a semiconductor substrate or wafer. In the manufacture of such integrated circuits, it is desirable to periodically inspect the structure of these devices. In most cases, this is achieved by inspecting a cross-section of the wafer on which the integrated circuits are formed.
The conventional method for inspecting a cross-section of a semiconductor substrate generally includes the removal of material from the substrate in order to expose a cross-sectional surface. This is followed by the illumination of that surface by an imaging beam. The step of exposing a cross-sectional surface is typically accomplished by scanning the front surface of the semiconductor substrate with a focused ion-beam to excavate a trench. The vertical wall of this trench forms the cross-sectional surface to be imaged by the imaging beam.
The imaging beam is typically a particle beam that scans the cross-sectional surface exposed by the scanning focused ion-beam. The interaction of this particle beam with the cross-sectional surface results in the emission of charged particles. These charged particles are detected by a detector that provides data to a processor for transformation of the data into a cross-sectional image of the semiconductor substrate.
A disadvantage of the foregoing method of obtaining a cross-sectional image is that the cross-sectional surface is exposed point by point using a focused ion-beam. Because the ion-beam is focused to a small point, the source of ions must be one that is capable of providing a large ion current in a small area. Such sources, which are referred to as xe2x80x9cbrightxe2x80x9d sources, typically use, as a source of ions, a metal that is liquid at or slightly above room temperature. For brevity, we refer to such metals as xe2x80x9cliquid metals.xe2x80x9d Of these liquid metals, the one generally considered most practical, because of its high boiling point, is gallium.
Unfortunately, it is extremely undesirable to introduce stray metal ions, such as gallium, into the semiconductor fabrication process. The presence of even small amounts of metal ion left on a semiconductor substrate following exposure to a focused ion beam can, through diffusion, contaminate other circuits formed on that substrate. As a result, following inspection of the cross-section of a wafer, the entire wafer is routinely discarded.
It is therefore desirable in the art to provide a method and system for imaging a cross-section of a substrate but without the deposition of stray metal ions on the substrate.
A system according to the invention excavates a trench on the substrate by projecting an image of an aperture onto the substrate instead of by scanning a focused ion beam across the substrate. This relieves the constraint that the ion source be a bright source and opens the door to the use of an ion source other than a liquid metal source.
More particularly, in a system embodying the invention, the cross-sectional surface to be imaged is exposed by projecting an ion beam image of an aperture onto the front surface of a workpiece. This workpiece is typically a semiconductor substrate, or wafer. Because the ion beam source is not focused directly on the front surface of the workpiece, the constraints on the brightness of the ion source are considerably relaxed. In particular, in a system incorporating the principles of the invention, one can generate an ion beam with an ion source other than a liquid metal source. Because of the reduced risk of contamination by stray metal ions, the system of the invention is thus particularly suited for cross-sectional imaging of semiconductor wafers.
A system according to the invention includes a shaped-beam ion-projection column extending along a first axis. The ion-projection column illuminates the workpiece with an image of an aperture and thereby excavates a section of the workpiece having the size and shape of the aperture as projected by the ion-projection column, including any magnification factor. A vertical wall of the excavated section of the workpiece forms a cross-sectional surface that is then illuminated by a focused particle-beam generated by a focused-particle-beam column. The focused-particle beam column is oriented along a second axis that intersects the first axis at a predetermined angle. This predetermined angle can be either fixed or adjustable by a system operator.
The focused-particle-beam column can be either a scanning electron-microscope, in which case the focused particle-beam is a beam of electrons, or a scanning focused ion beam column, in which case the focused particle-beam is an ion beam.
The ion source in the shaped-beam ion-projection column can be a relatively low brightness source. Such low brightness sources are typically characterized by a brightness of less than 100,000 amps per square centimeter per steradian. These low brightness sources can be plasma sources, or surface plasma sources. However, the ion source can also be a liquid metal source such as a gallium source.
The shaped-beam ion-projection column can include a mask having an aperture in the shape of the region to be excavated. The mask is typically disposed between the ion source and the wafer surface. While the aperture can be any shape, it is preferable that the aperture have at least one straight edge for forming an easily imaged cross-sectional surface.
In practice, the edges of the aperture cannot be made perfectly smooth. In order to reduce the effect of imperfections in the edge of the aperture, the apparatus of the invention further includes an optional dithering system. Such a dithering system includes deflector plates disposed between the mask and the surface of the wafer. A time-varying voltage on these deflector plates causes a time-varying electric field through which the ions forming the image of the aperture pass on their way to the wafer surface. The periodic translation of the ion beam in response to the time varying electric field through which it passes results in a periodic variation in the location of the projected image on the front surface of the workpiece. This periodic variation in the location of the projected image averages out effects of imperfections on the edge of the aperture.
In another embodiment, a workpiece support holds a workpiece at a selectable angle relative to an ion beam generated by an ion-beam column. In this embodiment, the system switches between a cutting mode and an imaging mode. In the cutting mode, the workpiece support holds the workpiece such that its front surface is normal to the ion beam. The ion-beam column then focuses an image of an aperture on the front surface to excavate a section of the workpiece having the projected size and shape of the aperture. A vertical wall of the excavated section of the workpiece forms a cross-sectional surface. In the imaging mode, the workpiece support holds the workpiece at an angle relative to the ion-beam. In this mode, the ion-beam column scans a focused ion-beam along the cross-sectional surface and thereby forms an image of that surface.